1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of controlling the combustion parameters of an internal combustion engine in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being supplied in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being supplied in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
Some flexible fuel compensation control systems are designed to modify the gasoline fuel and spark calibrations based on input from a fuel composition sensor for optimum engine efficiency and performance. Other flexible fuel compensation control systems use a learned fuel alcohol content instead of a fuel composition sensor to adjust the gasoline calibration. These "sensorless" type control systems provide unique challenges for setting optimum engine operating parameters.
For instance, idle air control (IAC) enrichments may be too large when required transient enrichments for alcohol are applied to idle air control fuel in cold ambient conditions. Also, the required alcohol fuel during cold engine operation may not be the same for all engine loads at a given alcohol content and engine temperature. Furthermore, the alcohol injected fuel correction factors and cold ambient conditions may be largely different from an idle condition to a driving condition.
Therefore, it would be desirable to provide a method for providing the same level of engine performance when operating on alcohol fuels as for gasoline fuel for all driving conditions.